1. Field
Embodiments of the present invention relate to a mirror, a lithographic apparatus and a method for manufacturing a device.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. Recently, lithographic apparatus using extreme ultraviolet (EUV) radiation have been provided.
Some EUV sources, e.g. discharge produced plasma (DPP) or laser produced plasma (LPP) sources, emit radiation over a wide range of frequencies, even including infrared (IR), visible (VIS), ultraviolet (UV) and deep ultraviolet (DUV). These undesired frequencies, which may also be referred to as “further radiation”, may propagate and cause heating problems in the illumination system and projection system of the lithographic apparatus and may cause undesired exposure of the resist if not blocked. Although the multilayer mirrors of the illumination and projection systems are optimized for reflection of the desired wavelength, e.g. about 6.7 nm or about 13.5 nm, they may be optically flat and have relatively high reflectance for IR, visible and UV wavelengths.
It is, therefore, necessary to select from the source a relatively narrow band of frequencies for the projection beam. Even where the source has a relatively narrow emission line, it may be advantageous to reject radiation out of that line, especially at longer wavelengths.
EP Application No. 1 496 521, incorporated herein by reference in its entirety, describes a lithographic apparatus comprising a multi-layer mirror provided with protrusions constructed to form a one dimensional or a two-dimensional diffraction pattern. As a result, the EUV radiation passes through these protrusions without any substantial absorption, while the undesired further radiation is substantially blocked due to absorption, refraction or deflection of this further radiation when impinged on the protrusions.
It is a disadvantage of the known lithographic apparatus that in order to form diffraction patterns, the protrusions, which are individually manufactured elements, have to be provided with high accuracy. This may unnecessarily increase manufacturing costs of the known multi-layer mirror. In addition, the known multi-layer mirror includes areas not covered by the material of the protrusions, for example in regions between the protrusions. Such areas, when exposed to a reactive environment, for example to an H2 atmosphere of the lithographic apparatus, may be covered by contaminants. These contaminants may decrease reflectivity of the mirroring surface with respect to the EUV radiation and thereby degrade beam quality.